What PSU do I need for a GeForce RTX 4090?

Why the answer is a range

A GeForce RTX 4090 is rated at 450 W of board power, and NVIDIA recommends a 850 W supply for it. That single figure has to cover every processor anyone might pair with the card, which is why the honest answer depends on yours: across the 26 processors here this card needs anywhere from 850 W to 1000 W.

With a Ryzen 7 7800X3D the whole system draws 656 W at once — 450 W for the card, 162 W for the processor at its power ceiling, and 44 W for the board, memory, drives and fans. That calls for 850 W, leaving the supply at 77% of its rating rather than sitting on top of it.

The 150 W spread across processors is not a rounding difference. A Ryzen 5 at 88 W and a Core i9 at 253 W are 165 W apart before anything else is counted, which is more than a whole supply size. Picking from the table below rather than from the card's blanket recommendation is the difference between buying the right unit and buying two sizes too big.

On 10 of those pairings the arithmetic asks for more than NVIDIA's 850 W, because the processor is heavier than a blanket recommendation can assume. On 9 it asks for less, and the recommendation here stays at the vendor's number anyway — they know how their own board behaves when it spikes, and this calculator deliberately does not model that.